This invention relates to a power train which is particularly suitable for use in an amphibious vehicle capable of travel on land and water, and more particularly to an adaptation of a conventional automotive power train having an in-line engine and speed change transmission to drive both the rear wheels and the marine propulsion unit of an amphibious vehicle. The invention also relates to an amphibious vehicle having such a power train.
In a known automotive power train arrangement for a conventional land based vehicle, an engine and speed change transmission are positioned at the front of the vehicle in-line with the longitudinal axis of the vehicle. The driving end of the transmission faces the rear of the vehicle, and is connected by a drive shaft, otherwise known as a propeller shaft, to a rear differential for driving the rear wheels of the vehicle.
It is also known to use an in-line engine and speed change transmission to drive the wheels and marine propulsion unit of an amphibious vehicle. For example U.S. Pat. No. 4,958,584 (Williamson) discloses a power train arrangement in which the engine and transmission are located at the rear of the vehicle, with the driving end of the transmission facing the front of the vehicle. A propeller shaft provides drive from the transmission to a front differential for driving the front wheels of the vehicle. The rear wheels of the vehicle are not driven. A marine propulsion unit is positioned behind the engine, and is driven from the timing end of the crank shaft of the engine. This arrangement is best seen in FIG. 5 of U.S. Pat. No. 4,958,584.
U.S. Pat. No. 4,838,194 (Williamson) is the parent application to U.S. Pat. No. 4,958,584 above. This patent also discloses an amphibious vehicle having a power train arrangement in which an in-line engine and transmission appear to be located at the rear of the vehicle, with the driving end of the transmission facing the front of the vehicle. The front wheels are driven by a shaft from a transfer case which also provides drive to a rearward facing marine drive.
The marine drive has a long propeller shaft to a screw propeller, both of which, along with a rudder, can be raised for road use; and lowered for marine use. The arrangement leaves little room anywhere in the vehicle for an engine and speed change transmission; the location of which is not specifically disclosed.
If the engine and transmission are located behind the vehicle rear axle (as in the case in the continuation in part U.S. Pat. No. 4,958,584), they would either have to be located above the propeller shaft, raising the centre of gravity to the detriment of vehicle handling on land and water; or to one side of the propeller shaft, giving odd weight distribution, and packaging problems. Either of these options would need a skewed drive to the transfer box, leading to power losses and possible NVH (noise, vibration, and harshness) problems. It should also be noted that the long vertically adjustable propeller shaft would give rise to sealing problems in the bottom of the hull, which could lead to water ingress and corrosion problems in the transfer case. Altogether, this does not appear to be a practical layout for an amphibious vehicle.
European patent No EP 0 341 009 (Royle) shows a further example of an amphibious vehicle in which an in-line engine and transmission are provided at the rear of the vehicle, with the output of the transmission facing the front of the vehicle. In this layout, the transmission drives the rear wheels of vehicle via a drive shaft, whilst a marine propulsion unit, located behind the engine, is driven from the timing end of the engine.
There are significant disadvantages in the above known amphibious vehicle power train arrangements, especially in the light of the high demands which are required of a modern vehicle in road operation as discussed below.
When a vehicle accelerates in a forward direction, the front of the vehicle tends to lift upwards in reaction to the rotational acceleration of the wheels relative to the vehicle. This happens irrespective of whether the vehicle is front or rear wheel drive and can lead to a loss of traction between the front wheels and the road under acceleration. This problem is exaggerated in the known amphibious vehicle arrangements where the engine is positioned behind the rear wheels of the vehicle. This is because the weight of the engine when positioned behind the rear wheels adds to the lifting force; as opposed to a conventional power train arrangement with the engine at the front of the vehicle, where the engine weight would counteract the lifting force. Consequently, in the known amphibious vehicle arrangements, the front wheels will tend to lose traction under acceleration. In practice this causes excessive wheel spin and tire wear. This is a particular problem in the Williamson layouts in which the vehicle is front wheel drive.
Furthermore, when a vehicle leaves a bend, the adhesion between the tyre and road surface must resist both acceleration and centrifugal forces. If the combination of these forces approaches or goes beyond the tractive limits of the front tyres, an under-steer condition will occur. In the conventional land based vehicle power train arrangements previously mentioned, the weight of the engine is positioned at the front of the vehicle, which reduces the tendency to under-steer when cornering. However, in the known amphibious vehicle arrangements in which the weight of the engine is behind the rear wheels, there will be a reduction in the load on the front wheels which increases the likelihood of under-steer occurring.
It is also known to provide for four wheel drive capability in conventional land based automotive vehicles having an in-line engine and speed change transmission located at the front of the vehicle. In such arrangements, the output end of the transmission faces the rear of the vehicle and a transfer case is used to selectively drive the rear wheels only or the front and rear wheels.
It has been proposed to use an automotive power train of this type to drive an amphibious vehicle, using the rear wheel drive to drive the marine propulsion unit and the front wheel drive to drive the front wheels. In order to use the power train in this way, it is necessary to locate the engine and transmission in the conventional position, towards the front of the vehicle.
Owing to rearward weight bias requirement for travel on water in an amphibious vehicle, it has been found unsuitable to locate the engine and transmission in this conventional position. It has also been found, contrary to the teachings in the Williamson patents, to be unsuitable to drive only the front wheels of an amphibious vehicle from a rear or mid-mounted engine, because the weight of the engine is not over the driving wheels. This limits traction on wet slipways, leading to problems leaving water; and can lead to wheel spin and rapid tyre wear on the road.